1. Field of the Invention
This invention relates to well logging methods and apparatus for determining characteristics of the earth formations surrounding a borehole by irradiating the formations with neutrons and measuring the resulting spatial distribution of the neutrons within the formations. More specifically, the invention relates to a technique whereby inaccuracies in the determination of the formation characteristic as a result of standoff from the borehole wall or other environmental effects in the borehole, such as cement and casing, may be substantially eliminated.
2. Description of the Prior Art
Knowledge of the porosity of the earth formations surrounding a borehole is of fundamental importance in the petroleum industry. Reliable determinations of porosity are needed not only to identify possible oil or gas producing horizons, but also to calculate other important parameters, such as the maximum producible oil index of a specific formation. In a conventional technique described in U.S. Pat. No. 3,566,117 issued to M. P. Tixier on Feb. 23, 1971, the porosity, commonly referred to as the "neutron porosity", derived from a neutron-neutron logging tool may be compared with the value of porosity, commonly referred to as the "density porosity", derived from a gamma-gamma formation density logging tool of the type disclosed in U.S. Pat. No. 3,321,625 issued to J. S. Wahl on May 23, 1967 in order to detect the presence of hydrocarbon gas in the formation.
Porosity measurements are generally performed by a dual-detector neutron porosity logging tool provided with a neutron-emitting source that irradiates the formation under study. The tool is typically urged against one side of the borehole wall by tool eccentralizers. The resulting neutron population is sampled by a pair of neutron detectors spaced at different distances from the source. A tool of this sort is described in detail in U.S. Pat. No. 3,483,376 to S. Locke, patented Dec. 3, 1963. If the two-detector measurement is made at a sufficient distance from the source the effect of borehole size and tool standoff are minimized by taking the ratio of the counting ranges. A function former or equivalent system then conventionally converts the ratio into a signal that represents otherwise uncorrected formation porosity. Unwanted contributions to the "ratio porosity" may include contributions from elements of the environment of the investigation such as tool standoff, borehole size, mud cake thickness, borehole salinity, formation salinity, mud weight, etc. Correction of these environmental effects is subsequently conventionally accomplished in a separate operation by reference to a plurality of log interpretation charts. Such subsequent operations are a hindrance to on-site interpretation of the logging results. Clearly, it would be desirable to have a technique that would make "on-line" corrections, so that an on site continuous well interpretation could be conveniently made.
Unfortunately, full advantage of large source-detector spacing cannot be realized in practice. If the detectors are placed far enough from the source, the counting rate becomes unacceptably low. As a result, shorter source-detector spacings than the ideal are employed. This produces a tool response having reduced borehole and standoff effects but these effects are not completely eliminated. Experience has shown that attempts to decrease source-detector spacing in order to achieve higher counting rates result in a disproportinately greater change in the environmental influence on the far detector counting rate than on the near. In this event, the "ratio porosity" clearly is not free of borehole environmental effects.
Attempts have been made in the past to overcome the environmental effects disadvantage of neutron-neutron logging by making explicit measurements of the size of the environmental factor causing the effect and then making explicit corrections. U.S. Pat. Nos. 4,047,027 and 4,035,639 are exemplary of previous techniques that utilize this measurement and correction approach. While the technique disclosed in these patents has its merits, it has the disadvantage of requiring the additional apparatus necessary for making an explicit measurement of standoff. This being the case, it is not suitable for use with the current commercial neutron-neutron logging tools unless a major effort is undertaken to retrofit each tool with a tool standoff sensor. In addition, the porosity answer product will necessarily include an additional source of uncertainty due to the error of the standoff measurement.
Of all of the possible detrimental environmental effects, tool standoff is perhaps the most significant. In order to reduce the amount of time spent in the borehole during logging operations, recent practice has been to mate two or more logging tools together end to end. It will be recognized that this pracrtice of logging with combination tool strings tends to increase the likelihood that the neutral porosity tool will not closely follow the borehole wall but will tend to bridge over borehole irregularities created during the drilling operation so as to occasionally and unpredictably produce tool standoff.
Heretofore, most commercial dual detector neutron-neutron logging has been accomplished with thermal neutron detectors due to the fact that reasonable counting rate statistics are obtainable at source-detector spacings which yield values of porosity that are not too badly degraded by borehole environmental effects. However, because the measurement of porosity is based on the detection of thermal neutrons, the presence of thermal neutron absorbing elements in the formation or the borehole complicates the interpretation of the results. Such elements in the formation are commonly associated with clay and/or salt water. It is known how to derive information on clay types by means of logging tools that detect natural radioactivity. Unfortunately, the elements responsible for the natural radioactivity of clay are not the same as the thermal absorbers that interfere with the thermal neutron logging tool.
The importance of the influence of thermal neutron absorbers in the borehole or formation becomes apparent when it is understood that the earlier mentioned comparison of "density porosity" with "neutron porosity", in order to obtain an indication of hydrocarbon gas, becomes suspect where there are thermal neutron absorbers in the formation.
Thus a need is felt to turn to epithermal neutron detection which is insensitive to the presence of thermal neutron absorbers in the formation. Doing so is not as straight forward as one would like, however, due to the reduction in count rate and the consequent loss of statistics inherent in epithermal neutron detection as compared to thermal neutron detection. Attempts at improving the count rates and therefore the statistics of the measurement by moving the epithermal neutron detectors closer to the source have been frustrated by the deterioration of the porosity signal obtained by the near to far ratio technique of interpreting the data due to the increase detrimental influence of environmental effects such as tool standoff from the borehole wall and borehole size.
In a completed well, relative positions of casing and cement as well as the position of the sonde in the borehole have an appreciable effect on the determination of neutron porosity of the formation behind casing. It is very difficult to correct for this effect, since the exact geometry of casing and cement is rarely known. Thus, it would be desireable to have a neutron-neutron logging tool whose porosity determination of the formation behind the casing of a completed well is not subject to large uncertainites due to the nonuniformities of the casing and cement job in the completed well.
Reduction in count rate and the consequent loss of statistics also become a significant limiting factor on faster logging which is desireable in this age of increased urgency for the discovery of produceable hydrocarbons. Greater logging speeds would become possible by moving the detectors closer to the source if a suitable technique could be found to overcome the degradation of the determination of porosity that results from the enhanced effects of the environment of the investigation that occur when the detectors are moved closer to the source.
As previously mentioned, the dual detector neutron-neutron logging tool is normally run in an eccentered condition in order to minimize standoff effects. This requirement limits the tools with which the neutron logging tool may be combined to other tools that are also run in an eccentered condition. It would clearly be desireable for the neutron tool to be independent of standoff effects so that it could be combined with other tools normally run in a centered condition such as the dipmeter and sonic logging tools.
Finally, in some producing wells that have a tendency to sand, a conventional technique is to provide a "gravel pack" through which the well fluids are flowed. Such a gravel pack filters the produced fluid and prevents the sand particles from entering the production tubing. It would be of great value to have a logging technique that would enable determination of the location and vertical extent of the gravel pack in order to assure that the sanding zone has been adequately covered.